Bonnie's right, there's quite a lot of info about David Denny in Heylin's book. I think he was killed in some sort of road accident in Colorado - about 1980?

I would however exercise caution about the book - there seems to be, to be polite, a certain amount of spin put on Sandy's relationship with Trevor, Trevor's talent or lack of it, etc. He applies the same technique to other people, some of whom he interviewed for the book - not nice. I also think the book is partly to blame for the "doomed angel" image of Sandy which has grown up. She was NOT Janis Joplin (or Amy Winehouse for that matter).         

Mr Heylin was at the Sandy celebration at the Troubadour a few years ago, and had a boxful of hardback copies for sale - they were going for about 20 quid then. Paperback still available I think on Amazon, for about a tenner.